Thin film active matrix electroluminescent (EL) displays (AMELD) are well known in the art and are used as flat panel displays in a variety of applications. A typical display includes a plurality of picture elements (pixels) arranged in rows and columns. Each pixel contains an EL cell having an EL phosphor active layer between a pair of insulators and a pair of electrodes. Additionally, each pixel contains switching circuitry that controls illumination of the cell.
One example of a prior art AMELD is disclosed in U.S. Pat. No. 5,302,966, issued Apr. 12, 1994. The disclosed AMELD includes a switching circuit associated with each pixel for controlling application of a high voltage to the EL cell. Specifically, the switching circuit disclosed in the '966 patent comprises a first transistor having its gate connected to a select line, its source connected to a data line and its drain connected to a gate of a second transistor and through a first capacitor (pixel storage capacitor) to ground. The drain of the second transistor is connected to ground potential, its source is connected through a second capacitor to ground and to one electrode of an EL cell. The second electrode of the EL cell is connected to a high voltage alternating current source for exciting the phosphor within the EL cell. In addition to this specific circuit arrangement, the '966 patent discloses a number of other arrangements of transistors that can comprise the cell switching circuitry. Other switching circuit designs are disclosed in commonly assigned U.S. Pat. No. 5,587,329 issued Dec. 24, 1996.
In operation, as is well known in the AMELD art, the data and select lines are appropriately energized to cause a particular switching circuit, e.g., transistor pair, to apply high voltage to a particular EL cell. Once the voltage is applied to the cell, current passing through the EL cell causes the phosphor layer therein to become luminescent. The magnitude of the illumination is controlled by a pixel signal stored on the capacitor (pixel storage capacitor) within the switching circuitry.
In an AMELD having a high density of EL cells, electric fields generated by the high voltage applied to the EL cells interferes with the operation of the cell switching circuitry as well as neighboring cell switching circuitry. Specifically, the electric fields couple to the transistors and storage capacitor as well as the data and select lines that are located proximate an active EL cell. Consequently, in response to errant coupling of the electric fields, the EL cells may be activated or deactivated in error. Such electric field coupling also applies noise to the pixel storage capacitor that stores the pixel signal. Upon activation of the pixel, the noise may cause the pixel to illuminate incorrectly.
Therefore, a need exists in the art for an electroluminescent pixel that contains a conductive electric field shield between the EL cells and their associated switching circuitry such that the EL cells are isolated from the switching circuitry.